Drinking water for 23 million Californians. Lifeblood of our farm economy. Why it's so vital to save this Sacramento delta.

On this brisk, cloudless day, Tom Zuckerman and I are driving to his duck-hunting club on Rindge Tract, one of the low-slung rural islands that form the nucleus of California's Sacramento-San Joaquin Delta. With Zuckerman's two black Labradors kenneled in the back of his SUV, we bump along the rutted levee road that traces the curve of an inlet called Disappointment Slough. Below us lies a sunken cornfield, intentionally flooded after the harvest to attract migrating ducks like pintails and mallards. We pass an unused asparagus shed, but otherwise there's hardly a building in sight. A sign posted on a low fence warns visitors not to build outdoor fires: the soil is so rich in organic matter that it has been known to combust.

Once mostly tidal marsh, the 1,153-square-mile delta was tamed in the nineteenth century into isles of farmland laced with waterways. Rindge Tract, Zuckerman tells me, was co-owned by Herbert Hoover, who grew and processed spearmint here. As we drive, the bustle of northern California seems far away -- until a Panamanian cargo ship passes by at startlingly close range. "Right on time," says Zuckerman, a retired lawyer from an old delta farming family. The giant vessel glides by without a struggle, navigating a deepwater channel that leads to the landlocked Port of Stockton, 75 miles east of San Francisco and 50 miles south of the state capital, Sacramento.

It seems as if everywhere I drive in this inverted delta -- unlike conventional deltas, its broadest side faces away from the ocean -- there's another Escheresque twist or Roadside America absurdity. Huge cargo ships sail inland. Farms sit below sea level. Rivers run backward. The soil burns. Posters advertise a local SPAM festival. A hidden turnoff leads to Locke, a weathered rural Chinatown with a steakhouse called Al the Wops. The delta, along with San Francisco Bay, forms the largest estuary on the western coast of the Americas, yet for most Californians it remains unexplored and somewhat mysterious territory.

It is also territory of outsize importance. The delta serves as a vast switching yard for much of the state's water supply, including drinking water for 23 million people from the Bay Area to San Diego. Freshwater from its namesake rivers is channeled to two massive pumping stations, one owned by the state and the other by the U.S. Bureau of Reclamation.From the state facility, water enters a labyrinth of pipelines, tunnels, and canals, including the 444-mile-long California Aqueduct, that carries it to residential users. The federal pumps, meanwhile, divert water to the sprawling farms of the San Joaquin Valley, the core of U.S. fruit and vegetable production.

For all its value and beauty, though, the delta is also on the verge of collapse. Much of its land is kept artificially dry by 1,100 miles of jury-rigged levees that are inadequate to withstand a litany of growing stresses. First there's global warming, which could push sea levels two feet higher, or more, by century's end. Add to this the risk of flooding -- also linked to climate change -- as a result of increased rainfall and quicker snowmelt in the mountains. Finally, there's the growing chance of a devastating earthquake. Any of these phenomena could trigger a chain reaction of levee breaches, inundating farms and communities, displacing thousands of people, and sucking salt water deep into an already overstressed system. That, in turn, could leave Californians scrambling for freshwater for agriculture and residential consumption. In 2005 a respected study by the geologist Jeffrey Mount, director of the Center for Watershed Sciences at the University of California, Davis, and the environmental planner Robert Twiss added up the combined risks posed by earthquakes and floods and calculated a 64 percent chance that up to 20 levees will fail simultaneously within the next 50 years.

Some scientists draw parallels to the Gulf Coast just before Hurricane Katrina. "When I look at New Orleans and then turn and look at the Sacramento Delta, it's eerie," says Robert Bea, a professor of civil and environmental engineering at the University of California, Berkeley.

A more immediate crisis has already beset the delta, one that shows how deeply its ecological health and human welfare are entwined. Native fish populations -- salmon, steelhead, sturgeon, smelt -- are declining at such an alarming rate that the entire ecosystem appears to be in peril. Among the many culprits are the two pumping stations, which not only suck the fish into their machinery but also alter the region's underlying hydrology. The estuary's key indicator species, the delta smelt, is in such danger of extinction that in 2007 a federal judge limited the amount of water that could be exported from the delta during the months when the smelt was most vulnerable. San Joaquin Valley farmers, lacking sufficient water, say they let significant acreage go unplanted this year.

Those with a stake in the delta -- who live within its boundaries, study its wildlife, drink its water, or use that water for irrigation -- agree the place cannot sustain itself in its present state. That's where the consensus ends. Even basic scientific assumptions about the estuary's ecology are hotly disputed. So is the question of who should make the biggest sacrifices to rescue the delta, and California, from the brink of disaster.

Barry Yeoman is a freelance journalist based in Durham, North Carolina. In addition to OnEarth, his work has appeared in Discover; O, The Oprah Magazine; AARP The Magazine; and Audubon. His web site is www.barryyeoman.com.

OnEarth is published by the Natural Resources Defense Council. The opinions expressed by its editors and writers are their own and do not necessarily reflect the policies or positions of NRDC. Learn more.